Epidural injection of analgesics is a valuable adjunct to general anesthesia, and in dogs the lumbosacral intervertebral (LS) space is the preferred injection site. Not only are landmarks within this space easy to palpate, it is the largest access point to the epidural space and lowers the risk for spinal cord injury. Historically, the dog’s hindlimbs are pulled rostrally alongside the body in an attempt to increase the craniocaudal length of the dorsal interlaminar space between L7 and the sacral bone (the LS distance).
This study investigated whether rostral extension of the hindlimbs increased the LS distance in sternally recumbent anesthetized dogs. Computed tomography (CT) was performed first with the hindlimbs resting on the stifle and the feet extended posterior, then with the hindlimbs extended rostrally along the abdomen and chest. Measurements were taken and the CT images were interpreted. Ten of the 18 study dogs had evidence of ileo- and lumbosacral osteoarthrosis that was not detected clinically. The authors determined that positioning dogs in sternal recumbency with the hindlimbs extended rostrally significantly increased the LS distance as compared with leaving the limbs in a neutral position.
Commentary
This study used CT to accurately measure the LS distance, whereas previous studies estimated the distance based on other radiographic measurements. The results confirmed what many have been taught and suspected; namely, that flexing the lumbar spine via rostral extension of the hind-limbs increased the dorsal interlaminar distance more than placing the limbs in a neutral position. This makes it significantly easier to perform an epidural injection or place an epidural catheter at L7-S1. In addition, by using patient positioning to open the dorsal interlaminar space at L5-6, it is easier to obtain CSF from that space via spinal needle. —Mark Troxel, DVM, DACVIM (Neurology)
